The semiconductor substrate in large scale integrated circuit chips must be maintained at a predetermined voltage relative to the active portions of the circuit, in order to establish a suitable reference for the generation and processing of signals by the integrated circuit. The diffusion structures in the substrate are generally maintained in a reversed bias state and during normal operation they inject a small positive current into the substrate, known as the substrate load current or leakage load current. The large number of diffusion structures in a large scale integrated circuit chip substantially increases the magnitude of this load current.
Substrate voltage generator circuits are employed to pump negative charge back into the substrate of the integrated circuit chip in order to compensate for the substrate load current so as to maintain a suitable negative voltage bias on the substrate.
Existing substrate voltage generator circuits have difficulty in maintaining the substrate voltage within precise tolerances for large scale integrated circuit chips, especially when rapid changes in the substrate load current occur. A uniform and predefined substrate voltage generator value is essential to the proper operation of large scale integrated circuitry. Prior art substrate voltage generator circuits, for example as is described in U.S. Pat. No. 4,356,412, employ a feedback connection to control an oscillator which drives the charge pump of the circuit. The substrate voltage is controlled by gating the oscillator into either an on or off state in response to the voltage magnitude sensed on the substrate. This technique has typically resulted in less precise voltage tolerance in the resultant substrate voltage generated for large integrated circuit chips and has not provided a quick enough response to rapid changes which can occur in the substrate load current.